Preheat oven to 180˚C. Mix flour, baking soda and salt in a bowl.
Combine both sugars and the zest together until well mixed. Add butter and beat until pale and fluffy. Mix in the eggs one at a time, then the lemon juice. Gradually stir in the flour mixture until well combined.
Using a large spoon, scoop the dough onto grease-proof paper lined baking trays spacing about 5cm apart. Flatten each slightly and sprinkle with demerara sugar. Lightly brush with a wet pastry brush and sprinkle with more demerara sugar.